Local hardening

Sensitization acquired during one s lifetime can, nevertheless, subsequently disappear. In some patients patch tests which were once positive have become negative. Masons who have suffered from eczema due to cement do not necessarily undergo a relapse if they take up their construction work again (Hunziker and Musso, 1960). Is there undeniably a diminution of eczematous hypersensitivity Is this desensitization specific or not, is it localized (hardening ) or general, is it spontaneous or due to repeated contact with small doses of the allergen ... 

Local hardening, that is, induction, laser, and so forth, for medium-carbon steels... 

Local hardening None, assuming access None, particularly if done on-site Not less than about 100 mm... 

When plastic deformation occurs, crystallographic planes sHp past each other. SHp is fackitated by the unique atomic stmcture of metals, which consists of an electron cloud surrounding positive nuclei. This stmcture permits shifting of atomic position without separation of atomic planes and resultant fracture. The stress requked to sHp an atomic plane past an adjacent plane is extremely high if the entire plane moves at the same time. Therefore, the plane moves locally, which gives rise to line defects called dislocations. These dislocations explain strain hardening and many other phenomena. 

For thermoset-matrix materials, heat is usually added as a catalyst to speed the natural chemical reaction of polymerization. Two-part epoxies, such as found in your local hardware store, consist of a tube of epoxy and a tube of chemical hardener that react when mixed. Heat... 

The present review shows how the microhardness technique can be used to elucidate the dependence of a variety of local deformational processes upon polymer texture and morphology. Microhardness is a rather elusive quantity, that is really a combination of other mechanical properties. It is most suitably defined in terms of the pyramid indentation test. Hardness is primarily taken as a measure of the irreversible deformation mechanisms which characterize a polymeric material, though it also involves elastic and time dependent effects which depend on microstructural details. In isotropic lamellar polymers a hardness depression from ideal values, due to the finite crystal thickness, occurs. The interlamellar non-crystalline layer introduces an additional weak component which contributes further to a lowering of the hardness value. Annealing effects and chemical etching are shown to produce, on the contrary, a significant hardening of the material. The prevalent mechanisms for plastic deformation are proposed. Anisotropy behaviour for several oriented materials is critically discussed. 

Irregularities of a specimen s surface will result in local deformation with accompanying deformation hardening. This may lead to erroneous hardness numbers, although such errors may be small. 

---